The major objective of the present proposal is to further our understanding of the regulation of arginine and urea biosynthesis in mammalian systems and to identify the factors or mechanisms responsible for the coordination of ureogenesis with the other pathways of amino acid metabolism. To this end, the first enzymes in the urea pathway, carbamyl phosphate synthetase I and ornithine transcarbamylase, will be further characterized with respect to their physicochemical properties, molecular organization and multiple forms. A systematic approach based on the structure of the enzymes, free of possible effectors or modifying proteins will establish a basic framework on which to interpret kinetic behavior, substrate-binding, and interaction with metabolites in order to identify those factors which influence their in vitro activity. The role of N-acetyl-L-glutamate as a key factor in this process will be explored, both in terms of its mechanism of action in the activation of carbamyl phosphate synthetase, as well as its biosynthesis and breakdown.